A three-dimensional (3D) ordered open-cellular microstructure is an ordered 3D structure at the micrometer scale. Aspects of embodiments of the present invention are directed toward a functionally graded 3D ordered open-cellular microstructure and a method of making the same. Here, the term “functionally graded” refers to a spatial variation in the physical microstructure—and thus the properties—through the thickness of the material.
A 3D ordered open-cellular microstructure can be formed, for example, by using a stereolithography technique, which relies on a bottom-up, layer-by-layer approach. This process usually involves a platform (substrate) that is lowered into a photo-monomer (photopolymer) bath in discrete steps. At each step, a laser is scanned over the area of the photo-monomer that is to be cured (polymerized) for that particular layer. Once the layer is cured, the platform is lowered a specific amount (determined by the processing parameters and desired feature/surface resolution) and the process is repeated until the full 3D structure is created.
One example of such a stereolithography technique is disclosed in Hull et al., “Apparatus For Production Of Three-Dimensional Objects By Stereolithography,” U.S. Pat. No. 4,575,330, Mar. 11, 1986, which is incorporated by reference herein in its entirety.
Modifications to the above described stereolithography technique have been developed to improve the resolution with laser optics and special resin formulations, as well as modifications to decrease the fabrication time of the 3D structure by using a dynamic pattern generator to cure an entire layer at once. One example of such a modification is disclosed in Bertsch et al., “Microstereolithography: A Review,” Materials Research Society Symposium Proceedings, Vol. 758, 2003, which is incorporated by reference herein in its entirety. A fairly recent advancement to the standard stereolithography technique includes a two-photon polymerization process as disclosed in Sun et al., “Two-Photon Polymerization And 3D Lithographic Microfabrication,” APS, Vol. 170, 2004, which is incorporated by reference herein in its entirety. However, this advance process still relies on a complicated and time consuming layer-by-layer approach.
Previous work has also been done on creating polymer optical waveguides. A polymer optical waveguide can be formed in certain photopolymers that undergo a refractive index change during the polymerization process. If a monomer that is photo-sensitive is exposed to light (typically UV) under the right conditions, the initial area of polymerization, such as a small circular area, will “trap” the light and guide it to the tip of the polymerized region due to this index of refraction change, further advancing that polymerized region. This process will continue, leading to the formation of a waveguide structure with approximately the same cross-sectional dimensions along its entire length. The existing techniques to create polymer optical waveguides have only allowed one or a few waveguides to be formed and these techniques have not been used to create a self-supporting three-dimensional structure by patterning polymer optical waveguides.
In addition, functionally graded foams with random cell configurations have been fabricated through various techniques and are discussed in Lefebvre et al., “Method Of Making Open Cell Material,” U.S. Pat. No. 7,108,828, Sep. 19, 2006; in Lauf et al., “Method Of Making A Functionally Graded Material,” U.S. Pat. No. 6,375,877, Apr. 23, 2002; and in Vyakarnam et al., “Porous Tissue Scaffoldings For The Repair Or Regeneration Of Tissue,” U.S. Pat. No. 6,534,084, Mar. 18, 2003; the entire contents of each of which are incorporated herein by reference. However, these functionally graded structures with specifically designed and ordered microstructures rely on layer-by-layer approaches similar to the above discussed stereolithography techniques, an example of which is also discussed in Keicher et al., “Forming Structures From CAD Solid Models”, U.S. Pat. No. 6,391,251, May 21, 2002, which is incorporated herein by reference in its entirety. These above cited references do not, however, disclose or discuss a manufacturing technique for 3D patterning of self-propagating polymer optical waveguides to form functionally graded open-cellular microstructures.
As such, there continues to be a need for 3D patterning of self-propagating polymer optical waveguides to form functionally graded 3D ordered open-cellular microstructures (or open-cellular materials) on a large and useful scale using a simple technique.